1. Field of the Invention
The present invention relates to a communication signal generating device included in a communication apparatus for use in a communication system in which communication signals are exchanged among a plurality of communication apparatuses through a signal line.
2. Description of Related Art
As such a communication system, there are known ones having a network structure in which a plurality of communication apparatuses are respectively connected to a plurality of branch lines branched from a trunk line as a common signal line. However, the communication system having such a network structure has a problem in that there occur signal reflections at branch points (connection points) between the trunk line and each branch line, causing the communication signals to have ringing waveforms.
To cope with this problem, it is proposed to control the output impedances of the communication apparatuses outputting the communication signals. For example, refer to Japanese patent No. 3693877 (Patent document 1). The communication system disclosed in Patent document 1 includes digital signal output circuits configured to output signals to a communication line through their respective output buffers, and each of the digital signal output circuits is provided with an output impedance varying section disposed between its output buffer and the communication line. The output impedance varying section operates to shape the waveform of a signal outputted from the output buffer by varying the output impedance of the output buffer in accordance with variation of the level of the signal in order to reduce the high-frequency components of this signal.
However, the communication system as disclosed in Patent document 1 has a problem in that since each of the digital signal output circuits has be provided with the output impedance varying section in addition to the output buffer, the number of components constituting the communication system increases.